Synchrotron photoionization measurements of combustion intermediates: photoionization efficiency and identification of C3H2 isomers.

Photoionization mass spectrometry using tunable vacuum-ultraviolet synchrotron radiation is applied to the study of C3H2 Sampled from a rich cyclopentene flame. The photoionization efficiency has been measured between 8.5 eV and 11.0 eV. Franck-Condon factors for photoionization are calculated from B3LYP/ 6-311++-G(d,p) characterizations of the neutral and cation of the two lowest-energy C3H2 isomers, triplet propargylene (HCCCH, prop-2-ynylidene) and singlet cyclopropenylidene (cyclo-HCCCH). Comparison of the calculated Franck-Condon envelopes with the experimental photoionization efficiency spectrum determines the adiabatic ionization energy of triplet propargylene to be (8.96 +/- 0.04) eV. Ionization energies for cyclopropenylidene, propargylene and propadienylidene (H2CCC) calculated using QCISD(T) with triple-zeta and quadruple-zeta basis sets extrapolated to the infinite basis set limit are in excellent agreement with the present determination of the ionization energy for propargylene and with literature values for cyclopropenylidene and propadienylidene. The results suggest the presence of both propargylene and cyclopropenylidene in the cyclopentene flame and allow reanalysis of electron ionization measurements of C3H2 in other flames. Possible chemical pathways for C3H2 formation in these flames are briefly discussed.